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What is SLD? 
If you hear terms like freezing rain (FZRA),
freezing drizzle (FZDZ) or thunderstorms during your weather briefing,
you are also likely dealing with the risk of SLD. Despite
what some pilots will tell you, SLD does not stand for
supercooled liquid
drops; they are truly missing the point. It stands for
supercooled LARGE
drops.
What's a large drop? The drop
size definition of SLD is derived from the FAA certification standards
of ice protection systems (IPS). A manufacturer along with
the FAA's approval certifies such a system for flight into known icing
conditions. However, this certification only applies for
"small drop" icing conditions. Flight into large drop
conditions is strictly outside the certification envelope of the
aircraft.
Pilots are often astonished to find out
that a large drop isn't very large at all. A large drop condition based
on these certification standards is defined as an environment where the
median volumetric diameter (MVD) of the drops is greater than 50
microns. If you are micron-challenged, 1,000 microns equals 1
millimeter. To put this in perspective, the average human
hair is only 100 microns in diameter...that means a large drop is
anything greater than half the diameter of a human hair!
Essentially, once a drop becomes barely big enough to see with the
naked eye, it's a hazard to all
aircraft, even those with certified IPSs.
SLD can be divided into two primary categories,
convective and non-convective. Convective SLD occurs within
deep, moist convection - or more specifically, vertically-developed
cumuliform clouds (as shown to the left).
This includes towering cumulus (TCU) and cumulonimbus (CB)
clouds. Convective SLD is usually encountered during the warm
season at much higher altitudes. The best way to avoid
convective SLD is to remain outside of cumuliform cloud boundaries when
flying at or above the melting level (0°C). This is usually
easily accomplished since most convective SLD is often spotty since the
clouds are often spotty.
Non-convective SLD includes freezing rain (MVD
greater than 500 microns) and freezing drizzle (MVD between 100 and 500
microns) environments. This does not imply that the
temperature at the surface has to be below 0°C. In fact, it
doesn't imply that precipitation is reaching the surface either.
Both of these forms of SLD can occur aloft and
may remain aloft carried by upward mixing or may evaporate before
reaching the surface.
Non-convective SLD can be split into two categories:
classical freezing rain and non-classical freezing rain
(we'll bring freezing drizzle into the discussion in a bit).
Most pilots were taught about classical freezing rain. It is
usually produced in very deep saturated environments during the cold
season where the clouds aloft are producing snow. Pilots were
taught that this snow falls through the cold clouds to eventually fall
into a layer of air that is warmer than 0°C (called a warm nose).
In turn, this completely melts the snowflakes into raindrops.
Lastly, the raindrops fall into a subfreezing layer (usually
very close to the surface) to form freezing rain which is considered
SLD. This profile can be seen on the image above.
Click here to view a larger image.
Non-classical freezing rain is a bit more
challenging to understand. But, 92-percent of all freezing
rain events are non-classical. So it is important to
understand. The biggest difference is that the cloud top is
warmer (usually warmer than -12°C) and doesn't produce snow.
There is often a dry layer above the tops. The
result is an all-liquid process down toward the surface. The
temperature profile (shown on the left) can be very similar as the
classical freezing rain structure including the presence of a warm
nose. Click here to view a larger image.
However, the most significant
difference is that the non-classical freezing rain structure may not
have a layer above freezing (see right). In other words, the
entire temperature profile may be colder than 0°C, but there still is a
dry layer aloft and warm cloud tops. Click here to view a larger image.
Most of the time, this kind of profile will produce freezing
drizzle, not freezing rain. It is typically more shallow of
an environment, but can produce a hazard over a larger range of
altitudes because there's no warm nose that contains temperatures above
freezing.
Keep in mind that no aircraft is
certified into SLD conditions. While SLD does come in a
variety of intensities, avoidance is the best policy. If you
have an aircraft that's certified into known icing conditions, knowing
what you should avoid is paramount. A thermodynamic chart
such as the Skew-T log (p) diagram is priceless when it comes to
avoiding a nasty icing encounter.
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